1. Field of the Invention
The present invention relates to loudspeakers, and particularly relates to compression drivers and to phase plugs for compression drivers.
2. Description of Related Art
A compression driver is a type of loudspeaker in which an acoustically radiating diaphragm radiates acoustic waves into a small cavity. The cavity is connected by a phase plug (also known as a phase adaptor, a phase transformer, an acoustic transformer, etc.) to an aperture, which normally opens into a horn waveguide. The small cavity and throat area present the diaphragm with a high acoustic load, and because of this, it tends to be highly efficient. However, the cavity in front of the diaphragm can cause acoustic problems at high frequencies. In particular, the cavity can exhibit strong resonances (known as cavity modes) at distinct frequencies that are commonly within the working band of the compression driver. These resonances can undesirably introduce large pressure response variations in the output of the compression driver. Additionally, the high pressure levels in the cavity that occur when the resonances are excited are undesirable for driver linearity. The severity of the resonance problem is determined primarily by the shape of the cavity, the design of the phase plug and, more specifically, the location and size of the pathways (channels) through the phase plug.
The Journal of the Acoustical Society of America, Volume 25, No. 2, March 1953 (Bob H Smith of the University of California), discloses an investigation of the air chamber of horn type loudspeakers, which includes a method of calculating the positions and sizes of the entrances to the channels (pathways) in a phase plug with annular channels (slots). The aim of the disclosed method is to avoid the excitation of resonances caused by the motion of the air entering and leaving the channels in the phase plug. According to the mathematical analysis presented in that technical paper, in an ideal phase plug with annular channels, the widths of the channels should be very nearly the same irrespective of their radial position in the phase plug, but with increasing radial position the channel width should normally increase very gradually.
Whereas the technical paper by Bob Smith considers only the effect of the motion of the air in the channels, in reality resonances are also excited by the motion of the diaphragm itself. The present inventors have performed a new analysis including the latter effect, and have accordingly devised the present invention.